Looks like the WiFi mesh was the right solution for the horrendous WiFi coverage in the new flat.

3 little mesh routers later, and I have excellent speeds everywhere on both floors, and, more importantly in the office where I edit / publish stuff.

@thelinuxEXP Yes, a PLC does wonders. A lot more stable.
@dm29 Maybe in a brand new house or flat. On 1960s French power lines, it never works properly, way too many appliances and twists and turns in the electrical. PLC gave me 1/10th of the mesh WiFi speeds :)
@thelinuxEXP Yes, I don't really know how it works. Marvels me it just exists. I had not expected bare copper to transfer high speed internet. Surely an electrician could offer a simple explanation to us both!